The Government and the police have not made a convincing case to support the controversial bid to extend terror detention limits, MPs said.
Members of the Home Affairs Select Committee said the Muslim community could come to regard the rules as a form of internment if ministers failed to prove why a longer period is required.
Home Secretary Jacqui Smith has drawn up plans to extend the current 28-day limit to 42 days.
Opposition to a higher limit from the Conservatives, Liberal Democrats, some Labour backbenchers and civil rights groups could turn the proposals into Prime Minister Gordon Brown's toughest test so far.
Committee chairman Keith Vaz said: "We saw no evidence that there was a case for extending the pre-charge detention beyond 28 days. The Home Secretary herself told us that only six out of 71 responses to the Government's proposals on going beyond 28 days were supportive. This was a surprisingly small number."
While the threat from terrorism remained "real and acute", it could be counter-productive to raise the maximum time for which police can hold and question suspects, the report said.
Alternative changes to the law could make it easier to bring prosecutions and avoid the need to raise the limit, it added.
For example, the committee said it was "ridiculous" that Britain still bars the use of evidence from telephone taps and other bugs - known as "intercept" evidence.
The all-party committee said the Government should begin urgent discussions to reform existing powers - under the Civil Contingencies Act - which could bring about a temporary extension of pre-charge limits.
